In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is positive-working. Conversely, if the unimaged regions are removed, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. There is a desire for positive working, thermally imageable elements that can be developed in conventional alkaline developers and that are both bakeable and resistant to press chemistries, such as inks, fountain solution, and the solvents used in washes, such as UV washes. Bakeability is highly desirable because baking increases the press runlength. Thus, a need exists for thermally imageable elements that can be developed in alkaline developers, resist chemicals typically encountered in a press room environment, and can be baked in order to increase runlength.